Gut bacteria and the brain: Are we controlled by microbes?

The relationship between our brain and our gut has been the subject of research for many years, but it is getting deeper and deeper. One striking fact is that our minds are to some extent controlled by the bacteria in our gut!

Our gut has its own defense system against pathogens, but it also promotes the survival and growth of “healthy gut bacteria”. The vast majority of these single-celled visitors live in the gut. It is difficult to estimate the number of bacteria in our gut, but it is possible to say that about 40 trillion bacteria pass through it and that bacteria are the main component of feces.  Most of our gut bacteria belong to 30 to 40 species, totaling about a thousand different species. So, in real terms, we are more bacteria than humans.

Of course, bacteria benefit from the warmth and nutrients in our intestines. But it is not a one-way relationship; they also give back. Some species benefit us by breaking down dietary fiber into short-chain fatty acids that we can then absorb and use. They metabolize a number of compounds on our behalf and are involved in the synthesis of vitamins B and K. On the other side of the coin, the compromise of gut bacteria is an important factor in autoimmune system weakness and infections. The role of the microbiome in health and disease is slowly being demystified. The latest and perhaps most remarkable finding is the ability of gut bacteria to govern our brain and behavior.

Why are the gut and brain connected?

What happens in our gut is literally a matter of life and death. If there is a problem in our gut with processing nutrients and thus ensuring nutrient absorption, the brain needs to be informed about it. When our gut is under attack by a pathogen, it alerts the brain. The connection between our gut and brain is maintained on hormonal, immunological, and neural levels via the central nervous system and the enteric nervous system, which governs the function of the gut. This is also called the “gut-brain axis”.

In fact, almost everyone has experienced the connection between the gut and the brain. For example, when we are extremely stressed or anxious, our bowel movements accelerate.  From another perspective, the bacteria in our intestines affect our psychology and behavior. Constant constipation is associated with chronic fatigue or depression, for example.

Stress and the gut

In humans, the hypothalamic-pituitary-adrenal (HPA) axis is the primary responsive system to all forms of stress. It is one of the most important players in the limbic system and is heavily involved with emotions and memory. Stress activates the HPA axis and eventually leads to the release of cortisol, the “stress hormone”, which has various effects on many organs, including the brain and gut. The brain’s response to stress, therefore, has a direct effect on gut cells. On the other hand, these cell types are also under the influence of our resident bacterial army. Although the mechanisms by which the microbiota regulates the brain are less clear, there is evidence of a two-way dialogue. Various studies have also shown that the microbiome has an effect on our mood, especially depressive behavior. Another observation in this regard is that children with autism often have abnormal and less diverse bacterial communities in their gut. A study in 2004 noted that mice lacking gut bacteria had an exaggerated HPA axis response to stress.

On the other hand, stress is known to increase the permeability of the intestinal wall, which allows bacteria easier access to both the immune system and the neuronal cells of the enteric nervous system. This may be one of the ways in which bacteria affect us. Another study using foodborne pathogens has also proven that bacteria in the gut can directly activate stress circuits.

In another study conducted with 25 animals and 15 humans, subjects were given Bifidobacterium and Lactobacillus for four weeks. The researchers concluded that these probiotics were effective in improving depression, anxiety, autism spectrum disorder, obsessive-compulsive disorder, memory, and concentration difficulties.

As a result of all this research, we know that we are partly under the control of single-celled life forms. Perhaps we would do well to remember that bacteria predate us by billions of years and are likely to outlive our species by billions of years.